Hydrophobic-oleophobic functional materials are useful for a variety of applications, including restoration and improvement of architectural structures and urban infrastructure, industrial/anti-fouling, optoelectronics (photovoltaics, fibers, displays), automotive, textile, and household. The technology for imparting hydrophobic and oleophobic functionality to surfaces is mature, as evident by the wide variety of materials readily available for commercial and household use, many of which are based on fluoropolymers. The quality of water- and oil-repellency of materials is based on the chemical nature of the given material and its surface properties, with lower surface energies generally exhibiting non-wetting behavior.
Conventional hydrophobic-oleophobic and ultraphobic materials and surfaces typically suffer from low mechanical strength and/or low abrasion resistance. The weakness of PTFE, the most widely used hydrophobic-oleophobic material, is compounded by its low adhesive strength to substrates. To compound the problem, the damaged surfaces may be rendered even worse (more hydrophilic and easily wetted or soiled) than they were before functionalization. So-called superhydrophobic or self-cleaning materials represent the current state of the art when it comes to non-wetting behavior. These materials owe their properties to precise engineering of surface topography. Unfortunately, these materials may also not be adequately durable due to the fragility of the microstructured surface features.
To overcome these deficiencies, many researchers have turned to using sol-gel methods to apply a fluorinated organic monolayer to solid substrates or as monolayers atop hardened films. These may perform better than bulk fluoropolymers owing to the strong Si—O film-substrate bond achieved by sol-gel chemistry. Additionally, sol-gel may allow for high thermal stability and optical transparency. During film deposition the fluorinated organosilane moieties, which impart the low-energy surface, align at the solid-air interface because of their chemical nature—it is this very repellency that makes them useful. This surface stratification means that the functional moieties are not sufficiently incorporated into the bulk, so most, if not all, functional groups end up at the molecularly-thin region at the air interface. Although the chemical bond to the substrate is strong, the material as a functional coating still lacks durability because the surface monolayer is easily worn. Since they offer only transient protection, the ability of hydrophobic-oleophobic functional material to resist abrasive wear is an ongoing challenge that still prevents widespread use of these materials.
As such, it would be desirable to obtain self-cleaning materials that are more durable, robust and/or abrasion resistant that those currently available. Additionally, it would be advantageous to develop a coating that works independently of the substrate condition since it is not always possible or desired to texture the substrate surface